In a cellular telephone system, a "subscriber" or "mobile" radiotelephone terminal unit, which may be an automobile-based, portable, or hand-held transceiver, having no fixed location communicates with a fixed, "base" radiotelephone station that is located in and acts as a communications "hub" for a particular and generally exclusive geographic region known as a "cell." Once communication is established between a given mobile terminal and base station and the mobile terminal is moved into another cell, communications with the terminal is "handed off" from the first base station to the base station for the second cell. Typically the base station is linked to a conventional local telephone trunk so that the mobile user can communicate with a conventional "wire line" (which itself may include radio transmission) telephone user from his mobile terminal.
The past decade has seen the rise of cellular telephones operating under protocols such as "AMPS" (Advanced Mobile Phone Service) or "TACS" (Total Access Communications System), respectively specified in the EIA Interim Standards IS-3-D, IS-19-A, and IS-20-A and in the United Kingdom Total Access Communications System: Mobile Station-Land Station Compatibility Specification. Generally, the implementation of these cellular telephone protocols allow two-way, fully duplexed communication between mobile terminals and base stations, each connected to a wire line, serving a limited geographic area and having the capability of handing off a communication to a second base station as the mobile terminal enters into the second base station's service area or cell. Each base station is allocated a set of numerous frequency pairs (voice channels), each pair to allow duplex operation, to allow simultaneous separate service of numerous mobile terminals. Control functions are generally communicated over a single control channel for a given cell. These include requests and acknowledgments from and to mobile terminals for call set-ups. Different voice and control channels are used as between adjacent cells. Typically, the base stations use one transceiver per channel, with the first transceiver on a rack being the one dedicated to the cell's control channel.
In normal cellular telephone operation, the base station is connected by wire line to a central switching complex, known as a "mobile telephone switching office," at which all call routing functions, including handing off, typically are performed. Thus, for example, a mobile terminal call addressed to another mobile terminal, even within the same cell, is directed by the base station to the mobile telephone switching office for routing back to the base station for connection with the addressee terminal.
There are situations in which normal cellular operation including the above-mentioned features is not available or even appropriate. As examples, thinly populated and traveled areas and radio isolated areas such as underground garages generally are not appropriate for full cellular service by expensive full-function base stations and digital switching equipment. Nonetheless, given the large number of cellular mobile terminals now in use, and, in many cases, installed in automobiles, it clearly would be a valuable extension of service of the mobile terminals to allow their use in situations in which full cellular service is not available.